When a high speed ratio is required, a 3 central gears type planetary gearing (3C type) is a better candidate than a worm gearing or a multi-stage gear reducer. Although the terminology used here may be different from those in various patents and books, in this type of planetary gearing there are always three central gears having the same common central axis namely a centrally located sun gear and a pair of surrounding ring gears located side-by-side. One or more planet gears are disposed between the sun and ring gears. As a practical matter, the 3C type planetary gearing is a combination of two single planetary gearings, which enables the 3C type planetary gearing to obtain a high speed ratio with a compact structure.
Many patents from the 1940's (for example, U.S. Pat. No. 240,1875) to recent years (for example, Jansson U.S. Pat. No. 4,366,727 issued Jan. 4, 1983, Rosen U.S. Pat. No. 4,280,376 issued July 21, 1981, Buuck U.S. Pat. No. 4,043,226 issued Aug. 23, 1977 and Tappan U.S. Pat. No. 4,040,312 issued Aug. 9, 1977) have been based on the principle of the 3C type gearing. However, in prior art of this type, each planet gear is a stepped or double faced gear comprising two different sized gears to mesh with the two ring gears of different sizes, and a carrier is usually indispensable to such 3C type and all other types of planetary gearing. The angular position between the two different sized gears on each stepped planet gear must be accurate, otherwise not all the planet gears can be mounted in. It is very difficult to keep those angular positions during manufacture. The carrier may be the most expensive part since the holes on it should be accurately equally spaced for assembling the planet gears in position and there are two bearings and one shaft for supporting each planet gear, and bearings for supporting the carrier. Size and cost have become important indices of design. Whether each planet gear can have a uniform tooth profile in the longitudinal direction instead of the stepped gear and the carrier can be eliminated is of especial importance to the development of the planetary gearing.
For dynamic balance, a plurality of equally spaced planet gears is needed. If each planet gear is made of a uniform tooth profile in the longitudinal direction, it should be able to mesh with three central gears of different numbers of teeth, which may also incur different types of interference. When use only "one cylindrical planet gear" as claimed by Ito in U.S. Pat. No. 3,453,906, issued July 8, 1969, the chance of interference may be reduced. However, the mechanism is unbalanced, which results in a centrifugal force, uneven motion and noise. Therefore, "synthetic resin" gears are used to decrease "noise". The invention of using a number of "automatically floating" planet gears for sharing an "equalized load" is disclosed by Duggar Jr. in U.S. Pat. No. 3,675,510 issued July 11, 1972. The planet gears are "resiliently yieldable within the range of forces encountered in overcoming the interference fit at the time of initial assembly". However, he has not disclosed the following important questions: how much the initial interference fit should be applied at the time of assembly in order to make the planet gears float and take the equalized load at the time of working, how much the resilient yieldability the planet gears should have to overcome the interference fit, and how much additional torque and power should be increased to overcome the interference fit for making the gears to run. It might be very difficult to solve these questions, and this invention might be used for some special application, while power loss and uneven motion are of less importance. One of the fundamental requirements of gear-tooth action is to satisfy the basic law of gearing or law of conjugate gear-tooth action, which provides a uniform rotary motion by means of ger teeth. Not all the gears satisfy this law, for example the gears in either Ito or Duggar's invention. Meshing tooth by tooth, these gears can provide a constant average speed ratio. But during each revolution the speed ratio is changing resulting in uneven dynamic load, which has not been allowed in the most industrial use almost since last century after the law of conjugate gear-tooth action was formed in 1674. The unbalanced single planet gear in Ito's invention will cause a centrifugal force, which can become very large since this force is propotional to the product of the unbalanced mass and the square of velocity. Even if the gears are made of "synthetic resin" to decrease the mass and the velocity is supposed to be low, the unbalanced centrifugal force still changes the speed ratio during each revolution and the rotary motion can never be uniform. The resilient yieldable gears in Duggar's invention will change the tooth-form. Because once stress reaches yield point, the deformation is permanent and the original form can not be restored. In his invention "if more than one planet gear is used", then "the planet gears are stepped". Even if the stress is below the yield point and assume that the stepped planet gear can be modified to a gear with uniform tooth profile and the profile is correct (e.g. involute tooth profile), since the planet gears are "forceably inserted into mesh", the distorted tooth form can not satisfy the law of conjugation. Therefore, the gearing in either Ito or Duggar's invention might be used in some special cases, not for a general use, especially in industry.